Large vehicles commonly have a frame rear portion supported on a rear axle assembly at least partially through an air bag. Smaller vehicles are also starting to use this type of suspension. In many cases, swing arms are used to control the horizontal position of the frame relative to the rear axle assembly. The swing arm has an upper end pivotally connected to the frame and a lower end pivotally connected to the rear axle. An air bag extends primarily vertically between the axle assembly and the vehicle frame to serve as a spring that supports much of the weight of the frame on the axle. A vehicle manufacturer commonly sets a predetermined height for each air bag. A leveling valve is used to flow air into and out of each air bag to create an air bag pressure that results in the air bags remaining at the predetermined height. In one example, an air bag may have an optimum height of fifteen inches, and the pressure in each air bag may vary between 40 psi when the vehicle is empty, to 70 psi when the vehicle is fully loaded (e.g. to 75% of the maximum).
The height of an air bag previously has been sensed by a mechanical linkage between an axle assembly and an adjacent location on the vehicle frame. In the United States, it has been common to rely upon movement of a rod or other mechanical component connected to the axle assembly, and extending to a valve assembly mounted on the frame. Such movement directly opens and closes selected valves that respectively admit air from a high pressure source (e.g. 140 psi) to the air bags, or that dump air from the air bags into the atmosphere. In European vehicles, it is common to provide an electrical signal indicating the height of an air bag. This is accomplished by a mechanical linkage comprising a rod mounted on a vehicle axle assembly that operates a potentiometer mounted on the vehicle frame. The electrical signal is used to control valves that flow high pressure air into the air bags or that drain air from the air bags to the atmosphere, or environment.
Some disadvantages of a mechanical linkage are that it is usually thin and easily damaged, and has bushings that wear out. Also, a repairman may improperly adjust it, casing rapid wear of the vehicle transmission and poor vehicle suspension. Further, the air control valve may react instantly to road bumps and undulations, or short term vehicle acceleration and deceleration. Such reactions can cause excessive consumption of pressured air, and possibly compromise other systems such as the braking system that rely on pressured air. Apparatus for maintaining proper air bag pressure, without using a mechanical linkage between the lower end of the swing arm and the vehicle frame, would be of value.